Spatial and temporal runoff processes in the degraded Ethiopian Highlands: the Anjeni Watershed

Abstract

Abstract. As runoff mechanisms in the Ethiopian highlands are not well understood, performance of many soil and water conservation measures is inadequate because of ineffective placement outside the major runoff source areas. To improve understanding of the runoff generating mechanisms in these highlands, we monitored runoff volumes from 24 runoff plots constructed in the 113 ha Anjeni watershed, where historic data of rainfall and stream discharge were available. In addition, we assessed the effectiveness of charcoal and crop rooting depth in reducing runoff, in which we compared the effect of lupine (a deep-rooted crop) to that of barley. Daily rainfall, surface runoff, and root zone moisture content were measured during the monsoon seasons of 2012 and 2013 (with all plots being tilled in 2012, but only barley plots in 2013). In addition, long-term surface runoff (from four plots) and outlet discharge data from the research site (1989–1993) was analyzed and compared with our observations. Results showed that the degree of soil degradation and soil disturbance (tillage) were significant factors affecting plot runoff responses. As expected runoff was greater from more degraded soils, while tilled plots had greater soil storage and thus less runoff. Overall, barley plots produced significantly less runoff than lupine plots. Specifically, considerable difference was observed for smaller rainfall events (ca. <20 mm) in 2013, when lupine plots (non-tilled) resulted in greater runoff than barley plots (tilled). This suggests that plot rainfall–runoff relationships are greatly affected by root-zone storage, which is directly affected by soil degradation and tillage practices.